A Flight Management System or FMS is an onboard device, in this instance aboard an aircraft. It allows the computation of an onboard flight plan in the case of rerouting, notably. A flight plan is the detailed description of the route to be followed by an aircraft within the framework of a planned flight. It comprises notably a chronological sequence of waypoints described by their position, their altitude and their overflight time. The waypoints constitute the reference trajectory to be followed by the aircraft with a view to complying best with its flight plan. This trajectory is a valuable aid both to the ground control personnel and also to the pilot, for anticipating the movements of the aircraft, for example an aeroplane, and thus ensuring an optimum safety level, notably within the framework of the maintaining of inter-aircraft separation criteria.
There exist various flight management systems, depending on the type of craft and of application for which it is intended. However, certain elements are common to a majority of flight management systems. Indeed, a flight management system generally comprises a navigation database, a means for recording its flight plan and guidance means.
The navigation database contains all the information necessary for the computation of a flight plan, such as the airways, airports, runways or radio navigation data.
The flight plan recording means comprises an interface on which the pilot can enter all the information necessary for establishing his flight plan.
The trajectory to be followed can be calculated on the basis of the flight plan as well as the whole set of data liable to be useful to the pilot during the flight, such as the transit time at a waypoint, or an estimation of the quantity of fuel aboard.
One of the main tasks of a flight management system is to accurately determine the location of the aircraft. These systems must be capable of operating whatever the position of the craft on the terrestrial globe, commonly defined by a latitude and a longitude.
The calculation of a trajectory is carried out with respect to a true North, true North being a navigation term referring to the direction of the geographical North pole with respect to a given position.
When the current or estimated position of the aircraft is level with a pole, it then becomes impossible to define true North, true North being defined in all the directions or in no direction, depending on the calculation facilities used.
A first solution has been afforded, consisting in using the coordinates of the previous position of the aircraft outside of the polar region used by the flight management system to determine the direction of true North.
This solution makes it necessary to verify for each new position used by the management system whether the position is situated in a polar region, this presenting the drawback of complicating the calculation procedure since this verification requires additional steps.
Another solution has been described in the document by Watland (U.S. Pat. No. 5,448,486) for calculating the position of an aircraft or the angular deviation between the axis of orientation of true North passing through the instantaneous position and the axis passing through the instantaneous position and the estimated position of the aircraft.
When the instantaneous position of the aircraft is situated in a polar region, the invention consists in aligning the polar axis in the plane of the equator. In the embodiment described in the Watland document, the former geographical North pole is called the “Sumatran pole” and is situated at a latitude of 0° and a longitude of 90°.
In another embodiment disclosed in the Watland document, the latitude of the “Sumatran pole” is situated beyond a latitude of 45°, the position error thus incurred being less than 1%.
The solution proposed by Watland presents the drawback of being required, for each of the positions considered, to verify whether the latter is situated in a polar region, and of pivoting the reference frame such as described previously, if appropriate. This solution complicates the positioning calculation and trajectory calculation method.